Electrical compensation for tone distortion in electromechanical engraving



Out 12, 1943. A. s. GANO ELECTRICAL COMPENSATION FOR TONE DISTORTION IN ELECTROMECHANICAL ENGRAVING Filed March 31, 1942 2 Sheets-Sheet 1 IN VEN TOR.

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Patented .Oct. 12, 1943 ELECTRICAL COMPENSATION FOR TONE DISTORTION ENGRAVIN G IN ELECTROMECHANICAL Alfred S. Gano, Rochester, N. Y., assignor to Press Association, Inc., New York, N. Y., a corporation of New York Application March 31, 1942, Serial N0. 436,986

6 Claims.

This invention is concerned with improvements in the engraving of printing plates with special relation to systems by means of which an electromechanically engraved plate is produced by a transmitted signal.

An object of this invention is to provide in a transmitting circuit for a system of this type, means for automatically compensating for the tone distortion developed in the system which results in a departure from the desired relation between the light reflecting or'transmitting properties of corresponding areas of an original positive or negative and the print from a plate so engraved.

Other and more detailed objects of this invention will be apparent from the following disclosure which includes as part thereof the accompanying drawings.

This invention resides substantially in the combination, construction, arrangement and relative location of parts, as will be described in detail below.

In the drawings,

Figure 1 is a diagrammatic illustration of the transmitting and receiving equipment of a systern for producing printing plates by electromechanical engraving;

Figure 2 is a circuit diagram of the electrical distortion compensating element of the system of Figure 1;

Figure 3 is a characteristic curve illustrating the typical relation between the percentage area cut away by the engraving tool and the signal current operating the engraving tool; and

Figure 4 is a curve which is the inverse of that of Figure 3 and shows the required relation between the signal from the compensatin circuit to the signal to the compensating circuit to correct for the tonal distortion in the reproduction which would result from the use of the typical system without compensation.

In this disclosure the expression tone distortion refers to the departure from the desired relation between the light reflecting or transmitting properties of corresponding areas of an original positive or negative and a reproduction thereof printed from an electro-mechanically engraved plate reproduced from the positive or negative and cut by a typical engraving system. At this point it may be noted that, as will become apparent to those skilled in the art from the following disclosure, that the subject matter of this invention may be applied in systems in which the printing plate is produced thereby utilizing the reflected light from a positive print or the transmitted light from a negative. In other words, signal currents representative of a scene, as is well known, may be produced by a scanning system actuated by light reflected from or transmitted through an original. Likewise it will be apparent that the subject matter of this invention is in no way limited to the particular material comprising the plate to be engraved. Of course, in the case of halftone engravings the reproduction is in the form of a positive printed from the engraved halftone plate.

In the production of a halftone plate whether by etching, engraving or otherwise, the area of the plate is subdivided into a plurality of elemental areas which have the dimensions of the particular screen employed. For example, in the case of a No. 60 screen the smallest area to be considered is a inch square. For the purpose of making a density comparison between the original and the reproduction such a unit area will be considered. The tonal values between black and white, that is the halftones, result from the combination in such an elemental area of the inked and uninked portions. These areas are sufliciently small so that the combination appears as a single area of intermediate tone when viewed in the normal manner. The reflection factor of such an elemental area is the same as the reflection factor for the paper multiplied by the percentage of the area uninked plus the reflection factor for the inked portion of that area multiplied by the percentage of the area inked. A good working approximation may be had by neglecting the reflection from the inked portion. The reflection factor of a halftone screen area is then equal to the reflection factor of the paper multiplied by the percentage of the area not inked.

The desired reproduction characteristic is that in which the density of the reproduction referred to as maximum reflection is proportional to the density of the picture being copied. The proportionality constant is equal to the density range of the original. The maximum reflection of the reproduction is controlled by the paper reflection, and the minimum size of the small high-light inked dots. The minimum reflection is controlled by the blackness of the inked paper and the small areas not inked to maintain the screen effect.

The initial step in the process of producing an en raved printing plate is the production of an electrical signal current corresponding to the density of the picture being copied. This electrical signal is proportional to the reflection factor of the picture being copied or the light transmitting factor of the negative being copied. This signal after transmission either to a near or remote point is impressed upon the cutting device to actuate its cutting tool. In this invention, as in usual practice, the cutting tool is caused to traverse the plate to be engraved in synchronism with the movement of the scanning light which in traversing the original produces the electric signal current referred to. There are a number of known methods of cutting which will form a halftone screen when, of course, the cutting tool is subjected in addition to the signal current to a second current of the proper frequency, that is to what is termed a screen frequency. This screen frequency causes the cutting tool to vibrate at right angles to the plate at the required rate to produce in relation to the moving plate the selected screen size.

In any of these systems the percentage of each screen area which is cut away is controlled by the signal current and will vary with the signal in a manner characteristic of the particular system employed. If this characteristic results as is usually the case in tone distortion as defined above, correction is made in accordance with this invention by introducing into the electrical transmission circuit means for establishing a simple relation between its input and output currents to compensate for that distortion.

In mechanical engraving systems for producing printing plates the area cut away is determined, among other things, by the depth of the cut of the engraving tool and the angle of its cutting edge. Since the tool angle is constant the area cut away is controlled by causing the signal current to vary the depth of out. which variation, of course, is superimposed upon the periodic variation employed to produce the screen effect. In the shadow regions the cutting tool leaves the plate for each cycle of the screen frequency. The result is,a series of separated cutout areas. In this region the area cut away varies approximately as the square of the signal. In the highlight region the cutting tool remains in the plate at all times so that the tool in making a cut overlaps or passes over some of the area that was cut on.the preceding stroke. The result is that the area cut away varies less than in proportion to the signal. Those characteristic conditions which result in tone distortion are illustrated by the typical curve of Figure 3. There are other variables which infiuence the shape of this characteristic, but this invention is not so much concerned with the components which cause the overall tone distortion as it is with means for correcting it. For this reason further analysis of the cause of distortion in electro-mechanical engraving systems need not be inquired into.

Figure l diagrammatically illustrates a system of the type under discussion. The illustration discloses a system suitable for preparing a printing plate from a positive picture i which is mounted upon a movable support 2, which may be caused to move alternately in a single plane in directions at angles to each other to ultimately effect complete coverage of the picture. Associated with the movable support is a scanning device 3 which is diagrammatically illustrated as including a modulated light source and lens system 4 for focusing a spot of light on the picture I. Disposed in proper position is a photoelectric cell 5 which receives the light reflected from the spot illuminated and converts it into a representative signal current. The output of the scanning device is connected to the input of a suitable amplifier which is preferably of the 'tive lead 2 I.

vacuum tube type, whose output in turn is connected to the input of the compensating device I, which will be described in full detail. The output of the compensating device is connected to the transmission line 8, which may be short or long as conditions require. The transmission line feeds a suitable amplifier and demodulator which is likewise preferably of the vacuum tube type. This in turn feeds a direct current ampliher in, the output of which is connected to the operating magnet 12 of the cutter i i. The screen frequency oscillator I4 is connected to the cutter so as to superimpose upon the magnet i2 the screen frequency current. At I3 is diagrammatically illustrated the cutting tool which is actuated by the magnet l2 to convert the effects of the signal and oscillator currents into vertical movements of varying depth. The plate iii to be engraved is mounted upon a support l5 similar to the support 2. The support I5 is caused to move in synchronism with'the support 2 so that the cutting tool progressively covers the area of the plate 46 in synchronism with the movement of the scanning device for the transmitted picture. This synchronism of movement between the two supports 2 and i5 is indicated by a broken line and arrow heads, It is, of course, understood, as is well known in the art, that the movement of the support I5 is synchronized with the oscillator l4 so that the cutting tool will begin each series of cuts in proper relation with those previously made. It should be noted, as will be apparent to those skilled in the art, that the point in the transmission system at which the compensating device I is connected may be varied. Thus, while it has been indicated as associated with the transmitting equipment, it can equally well be associated with the receiving equipment.

The compensator circuit is shown in detail in Figure 2. The output of the amplifier 6 is fed to the primary of the input transformer ll of the compensator. The secondary of the transformer i1 is shunted by a potentiometer l8, the common terminals of which are connected to the grounded or negative side 2| of the circuit. The movable contact of the potentiometer I8 is directly connected to the grid of a triode IS. The cathode of this triode is provided with a suitable bias resistor 20. The plate circuit of the triode l9 includes a resistor 22 and the primary of the coupling transformer 23. The secondary of this transformer is connected to the plates of a double diode 24 to provide a full wave rectifier. The midpoint of the secondary of transformer 23 is connected to the wire 2|. The common point of resistor 22 and the primary of transformer 23 is coupled through a condenser 26 to one terminal of the primary of a transformer 28 which is shunted by a resistor 21. The secondary of this transformer is connected to the plates of a double diode 29 to form a full wave rectifier. The midpoint of the secondary of transformer 28 is connected through a battery 30 to the oathode of the diode 29 for biasing purposes The common terminal of the resistor 21 and the primary of transformer 28 is directly connected to the gridof a triode 3| whose cathode is connected through a potentiometer 32 to the nega- The potentiometer is bridged by a by-pass condenser 33. Th movable contact of the potentiometer 32 is connected through an adjustable resistor 25 to the cathode of the diode 24. A resistor 35 is connected between the grid of the triode 3| and the ground lead 2|. The plate of the triode 3| is connected to the prifrom. its novel subject matter.

mary of the output transformer 34, whose secondary is connected to the transmission circuit 8.

In the operation of thi circuit the carrier or signal voltage from the amplifier B is applied through the coupling transformer I! to the input of the triode I9 which 'acts as an amplifier. The output of this amplifier is impressed across th resistor 22 and the primary of transformer 23. Potentiometer 32 is so adjusted that the diode 24 is biased to a non-conducting condition for signals below a certain value which are represented on the curve of Figure 4 as the values below $1. For low values of signal current where the diode 24 is non-conducting the primary impedance of the transformer 23 is high compared to that of the resistor 22. A large proportion of the output voltage of the triode I 9 i therefore impressed on the primary or transformer 23. For values of voltage exceeding the value S1 the diode 24 becomes conducting which lowers the impedance of the transformer 23. The result is that a smaller fraction of the output voltage of i the triode I9 is impressed upon the primary of a this transformer.

The signal voltage developed across the primary of transformer 23 is applied to the grid of the triode 31 through the voltage divider arrangement, comprising the parallel relation of the resistor 21 and the primary of the trans-- former 28, which is in series with the resistor 35. The biasing battery 30 is adjusted so that the diode 29 i non-conducting for signal voltages below a' suitable value such as represented by the value S2 in Figure 4, When the diode 28 is non-conducting the primary winding of transformer 28 has a high impedance compared to resistor .21. lhus the voltage reaching the grid of triode 3| is then determined by resistors 21 and 35. signal voltage exceeds the value S2 and diode 29 becomes conducting the primary of transformer 28 has a much lower impedance than that of the resistor 21. This causes a larger proportion of the signal current to be applied to the grid of the output triode 3|. Thus the curve of Figure 4 undergoes a sharp upward bend for conditions of high signal value. The resulting characteristic transmission curve of the compensator can therefore be predetermined to produce a curve which is the inverse of the distortion curve of Figure 3. Thus a print reproduced from a plate engraved under the influence of such a compensating circuit will not accept the tonal distortion secondary characteristic of such a system. The operating conditions, of course, can be adjusted by means of the potentiometers I 8, 25 and 32, and by the adjustment of the biasing battery 30 to predetermine the position of the curve of Figure 4 and the points of the illustrative values 31 and S: on these curves.

From the above description it will be apparent to those skilled in the art that the subject matter of this invention is capable of variations within the object thereof without departure I contemplate, therefore, that the variations which become possible in the light of this disclosure are within the scope thereof, and particularly of the claims granted me.

What is claimed is:

1. In a system for making an electro-mechanically engraved printing plate, the combination including scanning means for producing currents representative of the scene to be engraved, of a When, however, the value of the plate, an electro-mechanical engraver, a transmission circuit coupling said scanning means with said engraver, and electrical compensating means in said transmission circuit for changing the signal currents impressed upon said engraver to compensate for the characteristic distortion inherent in said engraver.

2. In a system for producing an electromechanically engraved printing plate, the coml bination comprising means for producing signal currents representative of a scene to be engraved,

electro-mechanical engraving means, a transmission circuit interconnecting them wherein the overall operating characteristic is non-linear, and

variable impedance means in said transmission circuit having a characteristic which is inverse of said first characteristic.

3. The combination with a system for producing electro-mechanically engraved printing plates comprising means for producing signal currents representative of elemental areas of a picture to be reproduced, electro-mechanical engraving means, a transmission circuit interconnecting them, and electrical means in said circuit controlled by the signal for modifying the signal supplied to said engraving means so that a print produced from an engraved plate will be corrected for tone distortion due to the overall distortion characteristic of said system.

4. In a system of the type described the combination with means for engraving printing plates, and means connected thereto for generating and supplying signal currents representative of the scene to be engraved, of means actuated by said currents for modifying them to correct for the non-linear characteristic of said engraving and actuating means whereby a print from an engraved plate will have tone separation proportional to that of the original scene.

40 5. The combination with a system for producing electro mechanically engraved printing plates comprising means for producing signal currents representative of a picture to be engraved, electro-mechanical engraving means and a transmission circuit interconnecting them, of compensating means in said circuit comprising a pair of multi-electrode vacuum tubes connected in tandem, each of said vacuum tubes having an input and an output circuit, and means coupled to the output circuit of said first tube for creating a high impedance in its output for signal currents below a predeterminedvalue whereby distortion in the shadows due to the non-linear characteristic of said system is compensated.

6. The combination with a system for producing electro mechanically engraved printing plates comprising means for producing signal currents representative of a picture to be engraved, electro-mechanical engraving means and a transmission circuit interconnecting them, of compensating means in said circuit comprising a pair of multi-electrode vacuum tubes connected in. tandem, each of said vacuum tubes having an input and an output circuit, means coupled to the output circuit of said first tube for creating a high impedance in its output for signal currents below a predetermined value, and means coupled to the input circuit of said second vacuum tube for producing a high impedance condition below a predetermined signal current value whereby the non-linear characteristic of said system is compensated.

AIJ'RED, S. GANO. 

